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HS-SPME-GC-MS analysis of onion (Allium cepa L.) and shallot (allium ascalonicum L.)

Authors:

Abstract

The volatile organic compounds of onion and shallot were determined via HS-SPME-GC-MS. The main components were dipropyldisulphide and allylpropyldisulphide. Thiopropanal S-oxide were detected only in onion volatiles. In shallot is interesting the presence of 2-methyl-2-pentenal, a compound with an intense fruity aroma, that can characterize the different aroma between onion and shallot. The SPME-GC-MS analysis of shallot after absorption on the SPME fiber at 50°C showed the presence of new compounds, whose structures have been discussed.
*Corresponding author.
Email: maurizio.dauria@unibas.it eISSN: 2550-2166 / © 2017 The Authors. Published by Rynnye Lyan Resources
Food Research 1 (5) : 161 - 165 (October 2017)
Journal homepage: http://www.myfoodresearch.com
FULL PAPER
HS-SPME-GC-MS Analysis of onion (Allium cepa L.) and shallot (Allium
ascalonicum L.)
*D’Auria, M. and Racioppi, R.
Dipartimento di Scienze, Università della Basilicata, Viale dell’Ateneo Lucano 10, 85100 Potenza, Italy
Article history:
Received: 19 May 2017
Received in revised form:
15 June 2017
Accepted: 16 June 2017
Available Online: 21 June
2017
Keywords:
Volatile organic compounds
Onion
Shallot
Solid phase microextraction
Gas chromatography
Mass spectrometry
DOI:
http://doi.org/10.26656/
fr.2017.5.055
Abstract
The volatile organic compounds of onion and shallot were determined via HS-SPME-GC
-MS. The main components were dipropyldisulphide and allylpropyldisulphide.
Thiopropanal S-oxide were detected only in onion volatiles. In shallot is interesting the
presence of 2-methyl-2-pentenal, a compound with an intense fruity aroma, that can
characterize the different aroma between onion and shallot. The SPME-GC-MS analysis
of shallot after absorption on the SPME fiber at 50°C showed the presence of new
compounds, whose structures have been discussed.
1. Introduction
Solid Phase MicroExtraction (SPME) has been
widely used to determine volatile organic compounds. In
our experience in this field, we used this method to
determine fire accelerant in fire debris (D'Auria, 2006),
volatile organic compounds in crude oil (D'Auria et al.,
2009), olive oil (Bentivenga, D’Auria, Fedeli et al.,
2004), wine (Acquaviva et al., 2014), saffron (D'Auria et
al.,2006), truffles (D'Auria et al., 2014), honey
(Bentivenga, D’Auria, Mauriello et al., 2004),
horseradish (D'Auria et al., 2004) thymus, rosemary,
laurel, sage (D'Auria and Racioppi, 2015), lavender and
oregano (D'Auria et al., 2005), kiwi (Celano at al.,
2006), Monilinia species (Mang et al., 2015) and in the
scent of an orchid (D'Auria et al., 2017).
The analysis of the volatile organic compounds
of onions has been the object of several works (Mondy et
al., 2001; Auger et al., 2005; Lanzotti 2013). SPME has
been used in the analysis of onion (Järvenpää et al.,
1998; Mondy et al., 2002; Soto et al., 2015). However,
SPME has not been used in the determination of volatile
organic compounds of shallot (Allium ascalonicum L.).
In this article, we want to report the SPME
analysis of both onion and shallot in order to understand
the origin of possible differences in the volatile organic
compounds composition.
2. Materials and methods
Onion and shallot have been cleaned of the
outermost layers and finely cut with a knife. The analysis
was carried out immediately after cutting, using
approximately two grams for each species.
A 50/30-μm DVB/CAR/PDMS module (57328-U,
Supelco, Milan, Italy) was employed to determine
VOCs. SPME fiber was maintained over the sample in a
20 ml vial at 20°C et at 40°C for 20 min. Analyses were
accomplished with an HP 6890 Plus gas chromatograph
equipped with a Phenomenex Zebron ZB-5 MS capillary
column (30-m x 0.25-mm i.d. x 0.25 μm FT) (Agilent,
Milan, Italy). An HP 5973 mass selective detector
(Agilent) was utilized with helium at 0.8 ml/min as the
carrier gas. A splitless injector was maintained at 250°C
and the detector at 230°C. The oven was held at 40°C for
2 min, then gradually warmed, 8°C/min, up to 250°C and
held for 10 min. Tentatively identification of aroma
162 D’Auria et al. / Food Research 1 (5) (2017) 161 - 165
eISSN: 2550-2166 © 2017 The Authors. Published by Rynnye Lyan Resources
components was based on mass spectra and Wiley 6 and
NITS 98 library comparison. Single VOC peak was
considered as identified when its experimental spectrum
matched with a score over 90% that present in the
library. The Kovats Retention Index was used to identify
the aroma components (Kovats, 1958). All the analyses
were performed in triplicate.
3. Results and discussion
The origin of sulfur-containing compounds in onions
has been extensively studied in the past (Block, 1992).
They originate from S-alkyl or S-alkenyl-L-cysteine-S-
oxide (1), present in the cytoplasm, when they react with
alliinase, located in the vacuoles. The reaction allowed
the formation of the corresponding sulfenic acids 2, that
was converted into the corresponding thiosulfinates 3
(Figure 1). This type of compounds are not stable and
was converted in several derivatives, some of them are
reported in Figure 1). In onions, a relative importance
was attributed to the propanethial S-oxide, a
lachrymatory factor found in onions.
We performed the analysis of both onion (Allium
cepa L.) and shallot (Allium ascalonicum L.) by
absorption of volatiles in sliced samples at 20 °C. The
use of this temperature has been due to the need to avoid
rapid decomposition of the lachrymatory agent emitted
by onion. The results are reported in Table 1.
The analysis of onion allowed the identification of
ten components. The main components were
dipropyldisulphide and allylpropyldisulphide. It is
noteworthy that, by using this type of analysis,
thiopropanal S-oxide, the lachrymatory agent emitted by
onions, can be detected. This compound cannot be
detected in the analysis of shallot. Also in shallot the
main components were dipropyldisulphide and
allylpropyldisulphide. It is interesting the presence of 2-
methyl-2-pentenal, an intense fruity aroma, that can
characterize the different aroma between onion and
shallot.
It is noteworthy that in previously published paper
on Allium cepa L. and Allium porrum L. 2-methyl-2-
pentenal was one of the main components found in the
analyzed oil (Schulz et al., 1998). Probably, this
observed difference depends on the extraction
methodology used in the article where an aqueous blend
was extracted with pentane for three hours.
Furthermore, it is noteworthy that most of the
compounds we found in both onion and shallot did not
compare in the list of the compound found in previous
SPME study on onion samples (Mondy et al., 2002).
When the absorption of volatile organic compounds
was performed at 50°C the nature of the compounds
found in onion and shallot considerably changed. The
results are collected in Table 2. We collected the results
only for shallot, in order to see if the increased
temperature can change the volatile profile of the
species. We observed a neat reduction of the amounts of
dipropyl- and allylpropyldisulphide, while the amount of
dipropyl and allyldipropyltrisuphide increased.
Furthermore, we observed the formation of new
compounds, in our knowledge never observed before.
FULL PAPER
Compound r.t.
[min] KI Area %
Onion Shallot
Methanethiol 1.61 500 3.25 ±
0.02
0.46 ±
0.01
Propanethiol 2.30 600 2.30 ±
0.01
4.20 ±
0.02
Thiopropanal S-oxide 4.37 740 0.36 ±
0.03
2-Methyl-2-pentenal 5.66 804 0.13 ±
0.01
2,5-Dimethylthiophene 7.18 865 2.33 ±
0.01
1.06 ±
0.01
Methylisopropyldisul-
phide
7.78 880 3.53 ±
0.03
2.74 ±
0.02
Dipropyldisulphide 11.64 1094 51.41
± 0.05
58.57
± 0.05
Allylpropyldisulphide 11.82 1098 20.69
± 0.05
13.27
± 0.05
Methyl propylthiosul-
fonate
12.35 1154 0.59 ±
0.01
0.46 ±
0.02
Dipropyltrisulphide 15.52 1294 4.66 ±
0.02
6.99 ±
0.03
Allylpropyltrisulfide 15.74 1309 1.31 ±
0.01
0.83 ±
0.01
Table 1. Volatile organic compounds in onion and shallot
with absorption on SPME fiber at 20°C.
RSCO2H
NH2
O
1 2 3
alliinase
RSOH RSSR1
O
H S
+
O-
Et
RS S SR2
O
R1
R
S
S
R1
O O
RSSSR1
RSSR1
Figure 1. The origin of sulphur compounds in onion.
163 D’Auria et al. / Food Research 1 (5) (2017) 161 - 165
eISSN: 2550-2166 © 2017 The Authors. Published by Rynnye Lyan Resources
Figure 2 showed the mass spectrum registered for the
compound 4 found at 19.41 min. The molecular peak
showed a very small abundance and can be found at m/z
214. The analysis of the relative abundance of the
isotopic peak at m/z 216 showed that this molecule
contained four sulfur atoms. The peak at m/z 117 can be
in agreement with CH3CH2CH2SCH2CH2CH2+ ion. The
assignment is in agreement also with the fragment at m/z
43 (CH3CH2CH2+) and that at m/z 75 (CH3CH2CH2S+).
On the basis of this assignment, the fragment at m/z 214
can be identified as (CH3CH2CH2SCH2CH2CH2SSSH)+..
The compound 4 can be tentatively identified as
described in Figure 3 as 3-(propanethiyl)propanetrithiol.
The compound 5, found after 19.64 min in our gas-
chromatographic analysis, shown the mass spectrum
reported in Figure 4. The spectrum did not show the
molecular peak. The fragment at m/z 115 is in agreement
with the presence of the structure CH3CH2CH2SCH=CH-
CH2+. The presence of a stabilized allylic carbocation
can account for the absence due to the loss of sulfur. The
proposed structure is shown in the Figure 3 as 3-
(propanethiyl)-2-propenetrithiol. The compound found at
19.80 min in the gas-chromatogram showed a molecular
peak at m/z 212 (Figure 5). The fragment at m/z 147 is
due to the loss of SSH, while the fragment at m/z 115,
can be due to the loss of SSSH from the molecular peak.
The mass is in agreement with the structure
CH2=CHCH2SCH2CH2CH2+. The proposed structure for
the compound 6 is shown in Figure 3 as 3-(2-
propenethiyl)-propanetrithiol.
The compound 7, found at 22.51 min, showed the
mass spectrum reported in Figure 6. The molecular peak
was not present in the spectrum. The main fragmentation
FULL PAPER
Compound r.t. [min] Area %
Propanethiol 2.31 2.57 ± 0.01
2-Methyl-2-pentenal 5.65 0.20 ± 0.01
2,5-Dimethylthiophene 7.18 0.51 ± 0.01
Methylpropyldisulphide 7.77 1.42 ± 0.03
Dipropyldisulphide 11.62 34.80 ± 0.05
Allylpropyldisulphide 11.79 7.14 ± 0.03
Methylpropylthiosulfonate 12.36 1.72 ± 0.02
Dipropyltrisulphide 15.66 21.70 ± 0.05
Allylpropyltrisulphide 15.82 9.18 ± 0.03
Compound 4 19.41 4.35 ± 0.03
Compound 5 19.64 2.00 ± 0.02
Compound 6 19.80 3.72 ± 0.02
Compound 7 22.51 2.02 ± 0.02
Table 2. Volatile organic compounds in shallot with
absorption on SPME fiber at 50°C.
Figure 2. Mass spectrum of compound 4.
S SSSH
4 5
S SSSH
S SSSH
S S S S
6
7
Figure 3. Proposed structures for compounds 4-7
Figure 4. Mass spectrum of compound 5
164 D’Auria et al. / Food Research 1 (5) (2017) 161 - 165
eISSN: 2550-2166 © 2017 The Authors. Published by Rynnye Lyan Resources
FULL PAPER
at m/z 149 is in agreement with a dimeric structure 7
(Figure 3) as 3,3'-di(propanethiyl)dipropyldisulphide.
In conclusion, we have shown that SPME can be
used in order to characterize the volatile organic
compounds emitted by sliced onions and shallots. We
observed some differences between these two species
attributable to the organoleptic differences between the
species. In fact, it is well known that shallot has less
intense, more aromatic and slightly garlicky smell.
Furthermore, the analysis obtained through
absorption on the SPME fiber at higher temperature
allowed to isolate new compounds.
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FULL PAPER
... Solid phase microextraction involves the use of specialized fibers coated with extraction polymer or adsorptive particles embedded in a polymer that capture volatile molecules for analysis by gas chromatography-mass spectrometry (GC-MS) and has been recently used to compare the composition of the volatile organic compounds of onion and shallot [7]. The solid phase microextraction fibers are specific for volatile organic compound analysis, making selection of the correct fiber essential for optimal component analysis. ...
... The GC method, adapted from Tocmo et al., was performed using a Thermofisher Trace GC Ultra system equipped with a fused silica TG-5MS (30 m × 0.25 mm i.d., 0.25 μm) column [7]. The injection port temperature was 250 °C, and the run method began with an initial temperature held at 50 °C for 3 min, raised at 3 °C/min to 150 °C, held at 150 °C for 3 min, and finally ramped at 25 °C/min to a final temperature of 250 °C, where the temperature was held for 5 min. ...
... The extraction solvents evaluated in this study were consistent with those reported for extraction of essential oils obtained from Allium genus specimens [7,10,12,45]. Colina-Coca et al. used headspace to determine organosulfide content of diced, freeze-dried, and pulverized onion. They used semiquantitation by dividing the compound abundance by the internal standard (allyl methyl sulfide) abundance rather than generation of a standard curve [46]. ...
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Qualitative and semi-quantitative analysis of organosulfides extracted from oil obtained by steam distillation of yellow onions was performed by gas chromatography-mass spectrometry (GC-MS). The extraction efficiency of organosulfides from onion oil was evaluated across four solvents: dichloromethane; diethyl ether; n-pentane; and hexanes. Analysis of solvent extracted organosulfides by GC-MS provided qualitative results that support the use of dichloromethane over other solvents based on identification of 27 organosulfides from the dichloromethane extract as compared to 10 from diethyl ether; 19 from n-pentane; and 17 from hexanes. Semi-quantitative evaluation of organosulfides present in the dichloromethane extract was performed using diallyl disulfide as the internal reference standard. Three organosulfides were detected in the extract at ≥5 mg/kg; 18 organosulfides between 3–5 mg/kg; and six organosulfides at <3 mg/kg. The E/Z isomers of 1-propenyl propyl trisulfide were among the most prevalent components extracted from the onion oil across all solvents; and 3,6-diethyl-1,2,4,5-tetrathiane was among the most abundant organosulfides in all solvents except hexanes. The method described here for the extraction of organosulfides from steam distilled onion oil surveys common solvents to arrive at a qualitative and semi-quantitative method of analysis for agricultural products involving onions; onion oil; and secondary metabolites of Allium spp.
... The SPME fibre used was divinylbenzene/carboxen/ polydimethylsiloxane (DVB/CAR/PDMS) 50/30 mm, StableFlex, 1 cm long, mounted to an SPME manual holder assembly (Supelco, Bellefonte, PA). This fiber has extensibility shown to be suitable for the volatile extraction compounds from tomato sauce preserved in different packaging materials (glass, tin and multilayer plastic containers) [20], Italian and Spanish ready-to-eat tomato sauces [8] and other vegetable matrices used as part of sofrito such as onion [21] and extra-virgin olive oil [22]. ...
... Hexanal is important in the fresh tomato flavour contributing to the green aroma. This aldehyde together with [21] 2-methyl-2-pentanal and benzaldehyde diminished in the cooking process [23]. Others like trans-2-heptenal, 6-methyl-5-hepten-2-one did not change significantly the percentage in all the samples [8,23]. ...
... On the other hand, several studies have reported that 2-methyl-2-pentenal, methyl-2-propenyldisulfide, 2,4-dimethylthiopene, di-2-propenyldisulfide, dipropyl disulfide, trans-propenylpropyldisulfide, di-2-propenyltrisulfide and 3,4-dihydro-3-vinyl-1,2-dithiin are volatile compounds linked with the presence of garlic and/or onion as ingredients in the tomato sauce [21,33,40]. In this sense, di-2-propenyldisulfide, methyl-2-propenyldisulfide, were present in CS1 and CS2 but not in CS5 which contains no garlic. ...
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... During this time, our research group used SPME in order to determine the composition of volatile organic compounds in a lot of natural matrices. Thus, we determined the composition of volatile compounds mixtures in olive oil [2][3][4], wine [5][6][7][8][9], saffron [10,11], fungi and truffles [12][13][14][15][16][17], honey [18], horseradish [19], thymus [20,21], origanum [20], lavandula [20], acinos [20], rosmarinus [21], laurus [21], salvia [21], actinidia [22], crude oil [23], onion [24], shallot [24], and beer [25]. Furthermore, recently, we used this methodology in the determination of the scent of a flower plant [26]. ...
... During this time, our research group used SPME in order to determine the composition of volatile organic compounds in a lot of natural matrices. Thus, we determined the composition of volatile compounds mixtures in olive oil [2][3][4], wine [5][6][7][8][9], saffron [10,11], fungi and truffles [12][13][14][15][16][17], honey [18], horseradish [19], thymus [20,21], origanum [20], lavandula [20], acinos [20], rosmarinus [21], laurus [21], salvia [21], actinidia [22], crude oil [23], onion [24], shallot [24], and beer [25]. Furthermore, recently, we used this methodology in the determination of the scent of a flower plant [26]. ...
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... During this time, our research group used SPME in order to determine the composition of volatile organic compounds in a lot of natural matrices. Thus, we determined the composition of volatile compounds mixtures in olive oil [2][3][4], wine [5][6][7][8][9], saffron [10,11], fungi and truffles [12][13][14][15][16][17], honey [18], horseradish [19], thymus [20,21], origanum [20], lavandula [20], acinos [20], rosmarinus [21], laurus [21], salvia [21], actinidia [22], crude oil [23], onion [24], shallot [24], and beer [25]. Furthermore, recently, we used this methodology in the determination of the scent of a flower plant [26]. ...
... During this time, our research group used SPME in order to determine the composition of volatile organic compounds in a lot of natural matrices. Thus, we determined the composition of volatile compounds mixtures in olive oil [2][3][4], wine [5][6][7][8][9], saffron [10,11], fungi and truffles [12][13][14][15][16][17], honey [18], horseradish [19], thymus [20,21], origanum [20], lavandula [20], acinos [20], rosmarinus [21], laurus [21], salvia [21], actinidia [22], crude oil [23], onion [24], shallot [24], and beer [25]. Furthermore, recently, we used this methodology in the determination of the scent of a flower plant [26]. ...
... After SPME, the fiber was withdrawn into the holder needle, removed from the vial and immediately introduced into the GC injector port for 6 min at 250 • C for thermal desorption of the VOMs. These analytical conditions were chosen taking into account a previous work that reported the analysis obtained through absorption in the SPME fiber at the higher temperature (40 • C) compared to 20 • C isolated new compounds in onion samples [16]. All assays were performed in triplicate. ...
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... To our knowledge, this was the first analysis of volatile sulfur compounds in A. cornutum. Interestingly, dipropyl disulfide was previously reported to be the predominant compound in A. cepa L. [28][29][30], but it is the compound that highly dominates in A. cornutum, as well, with 31.9% (Table 1). Colina-Coca et al. (2013) [31] identified 12 volatile sulfur compounds in processed onion, using HS-GC-MS. ...
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